1. Field of the Invention
The present invention relates to a method of preventing polymer scale deposition in a polymerization vessel during polymerization of a monomer having an ethylenic double bond.
2. Description of Prior Art
In processes of preparing polymers by polymerizing a monomer in a polymerization vessel, the problem that polymers deposit on the inner wall surface and other parts which come into contact with the monomer such as stirring blades of the polymerization vessel in the form of scale, is known. The deposition of the polymer scale on the inner wall results in disadvantages that the yield of the polymer and the cooling capacity of the polymerization vessel are lowered; that the polymer scale may peel and mix into manufactured polymer, thereby impairing the quality of the manufactured polymer; and that removal of such polymer scale is laborious and hence time-consuming. Further, since the polymer scale contains unreacted monomer, there is a danger that the operators may be exposed to the unreacted monomer, which may cause some physical disorders.
Heretofore, there are known methods for preventing polymer scale deposition in which the inner wall and so forth are coated with a suitable substance as a polymer scale preventive agent. Such substances suited as polymer preventive agents include, for example, particular polar compounds (Japanese Patent Publication (KOKOKU) No. 45-30343(1970)): dyes or pigments (Japanese Patent Publication (KOKOKU) No. 45-30835(1970)); aromatic amine compound (Japanese Pre-examination Patent Publication (KOKAI) No. 51-50887(1976)); a reaction product of a phenolic compound with an aromatic aldehyde (Japanese Pre-examination Patent Publication (KOKAI) No. 55-54317(1980)).
These methods are effective in preventing polymer scale deposition, in the case where the monomer to be polymerized is vinyl halide such as vinyl chloride or a monomer mixture containing largely a vinyl halide. However, in the case where the monomer having an ethylenic double bond to be polymerized is a monomer having the general formula: EQU CH.sub.2 .dbd.CXY
wherein X is a hydrogen atom or the methyl group; Y is a hydrogen atom or a group represented by the formula:--C.sub.n H.sub.2n+1 where n is an integer, normally an integer of 1 to 4, --COOH, --COOM where M is an alkali metal such as Na, K and Li or an ammonium ion, --COOC.sub.n H.sub.2n+1 where n is as defined above, --CN, --C.sub.6 H.sub.5, --C.sub.6 H.sub.4 Z (where Z is a hydrogen atom, --OH, --CH.sub.3 or --CH.dbd.CH.sub.2), --OCOC.sub.n H.sub.2n+1 where n is as defined above, --OC.sub.n H.sub.2n+1 where n is as defined above or --CH.dbd.CH.sub.2, including styrene, .alpha.-methylstyrene, acrylates and acrylonitrile, which have so high a dissolving power against the coatings formed by the above prior art methods, the coatings may be partly or completely dissolved away. Consequently, it becomes impossible to prevent the deposition of polymer scale effectively. Among the monomers, styrene, .alpha.-methylstyrene, acrylates and acrylonitrile have highly strong dissolving power against the coatings of the scale preventive agents; therefore effects of preventing polymer scale can not be attained as desired. Besides, particularly where a polymerization vessel made of stainless steel is used, polymer scale is liable to deposit on the inner wall surfaces of the polymerization vessel.